Ltk- aprt- mouse L cells were transformed to the tk+ phenotype with 10 nanograms of the herpes simplex virus thymidine kinase (tk) gene and 20 micrograms of several carrier DNA's (unlinked to the tk). Five transformant cell lines were studied in detail. Two lines were derived from transformations using circular pBR322 DNA as the carrier and three cell lines were obtained from transformations using circular SV40 DNA as the carrier. In all cases we were able to show that many of the restriction endonuclease fragments generated from the cellular DNA annealed to both tk and to pBR322 or SV40. In all of the cell lines some of these fragments also contain cellular DNA sequences. Thus it appears that when exogenous DNA's are introduced into these cells they are linked to each other before they are integrated into the cellular DNA. We have also been able to do a family study of contractural congenital arachnodactyly. The relationship of this condition to the Marfan syndrome, with which it can be confused, was studied by reviewing the reported incidence of the physical findings characteristic of both conditions. Criteria for distinguishing the two are being developed.